Field of the Invention
Embodiments of the present invention relate to a charged particle beam writing method and a charged particle beam writing apparatus, and, for example, relate to a method for adjusting magnification of an aperture image of an electron beam in the electron beam writing apparatus which irradiates a target object with electron beams.
Description of Related Art
In recent years, with high integration of LSI, the line width (critical dimension) of semiconductor device circuits is decreasing year by year. An electron beam (EB) writing technique which has excellent resolution is used as a method of producing an exposure mask (also referred to as a reticle) for forming a circuit pattern on such semiconductor devices.
FIG. 10 is a conceptual diagram explaining operations of a variable-shaped electron beam writing or “drawing” apparatus. The variable-shaped electron beam writing apparatus operates as described below. A first shaping aperture plate 410 has a quadrangular (rectangular) aperture 411 for shaping an electron beam 330. A second shaping aperture plate 420 has a variable shape aperture 421 for shaping the electron beam 330 having passed through the aperture 411 of the first shaping aperture plate 410 into a desired quadrangular shape. The electron beam 330 emitted from a charged particle source 430 and having passed through the aperture 411 of the first shaping aperture plate 410 is deflected by a deflector so as to pass through a part of the variable shape aperture 421 of the second shaping aperture plate 420, and thereby to irradiate a target object or “sample” 340 placed on a stage which continuously moves in one predetermined direction (e.g., x direction). In other words, a quadrangular shape that can pass through both the aperture 411 of the first shaping aperture plate 410 and the variable shape aperture 421 of the second shaping aperture plate 420 is used for pattern writing in a writing region of the target object 340 on the stage continuously moving in the x direction. This method of forming a given shape by letting beams pass through both the aperture 411 of the first shaping aperture plate 410 and the variable shape aperture 421 of the second shaping aperture plate 420 is referred to as a variable shaped beam (VSB) system.
In accordance with the generation (technical development stage) of pattern layout formed on the mask substrate used as a target object, a required dimension/size of a beam to be shot differs in the electron beam writing apparatus. If under the conditions that the maximum beam current is fixed, in the case of writing a mask including multiple micropatterns, it is advantageous to reduce the writing time by decreasing the maximum beam dimension and increasing the current density so as to secure a beam current even when the beam dimension is very fine. On the other hand, in the case of writing a mask including lots of comparatively large patterns and having a small number of shots as a whole, it is advantageous to increase the maximum beam dimension even if the current density is reduced. The reason is as follows. Between consecutive beam irradiations, the beam is blanked while the beam position is changed from one position to the next position on a mask. This blanking duration is called “settling time”. The cumulative settling time is part of a mask writing time. Therefore smaller number fo shots is favorable to achieve a shorter mask writing time.
Further, with respect to a plurality of patterns formed on the same mask, a required dimension/size of a beam to be shot differs between an actual circuit formation portion and a peripheral portion.
In the VSB electron beam writing, since the dimension of a beam irradiating a target object does not become greater than a dimension obtained by reducing the aperture dimension of the first shaping aperture plate 410 by a preset reduction ratio by the optical system, the beam dimension to be shot is restricted by the dimension of the aperture of the first shaping aperture plate 410. Therefore, in order to acquire a size, as a beam dimension, greater than a dimension obtained by reducing the aperture dimension of the first shaping aperture plate 410 by the preset reduction ratio, it is necessary to exchange the first shaping aperture plate 410 for another first shaping aperture plate having a larger aperture dimension. For performing this, there is a problem in that the electron optical column having been maintained in a vacuum state must be released in the atmosphere. Furthermore, changing the illumination optical system which illuminates the first shaping aperture plate 410 may be needed.
Such a problem may occur not only in the VSB electron beam writing but also in the electron beam writing for transferring an aperture image. Further, for example, the same problem may occur also in a multi beam writing apparatus since an aperture array image having passed a shaping aperture array irradiates, as multiple beams, the surface of the target object. Then, it is examined to alter the magnification of an aperture image by further arranging a lens paired with the objective lens. For example, although not concerning the VSB system, there is disclosed adjusting a reduction ratio finely by using a reducing lens paired with the objective lens while changing the height position of the surface of the target object in a transfer apparatus for reducing and transferring a mask image (e.g., refer to Japanese Patent Application Laid-open (JP-A) No. 5-175113). However, this technique just finely adjusts/corrects deviation of a reduction ratio to be a design value. Moreover, since the crossover position peculiar to an electron beam changes, the writing accuracy may be remarkably degraded.